Traditionally, bimetal temperature switches configured using a ceramic substrate as an insulative support for a thermostat have been proposed. (See, for example, International Patent Application Publication No. WO87/03137) FIGS. 1A, 1B, and 1C show an example of a conventional bimetal temperature switch that uses a ceramic substrate as an insulative support for a thermostat.
As shown in FIGS. 1A, 1B, and 1C, this bimetal temperature switch comprises a thin and rectangular support 1 made of alumina-ceramic. At the center of the support 1, a groove 2 is formed, and longitudinal ends of a bottom surface 1a are metalized.
Terminal tabs 3 and 4 are respectively fixed to these metalized longitudinal ends of the support 1.
Each of these terminal tabs 3 and 4 has a hole for soldering on one end, and the other end is divided three portions so as to form a fork shape The end comprises a pair of protrusions 6 on the outer sides, respectively, and a protrusion 7 at the center in such a manner that the pair of protrusions 6 and protrusion 7 great different vertical levels.
The pair of protrusions 6 at the lower level are jointed to the metalized ends on the bottom surface 1a of the support 1. The protrusion 7 that is at the higher level contacts the upper surface of the support 1.
A contact spring 8 has a hole 11 approximately at its center, and a pin 12 made of plastic is inserted into this hole 11. A head 13 of the pin 12 is engaged with the top plane of the contact spring 8, and a lower pole runs through a hole 14 at the center of a bimetal plate 15 and the groove 2 on the support 1.
The bimetal plate 15 is disposed between the support 1 and the spring 8. A collar 16 of the pin 12 is disposed between the contact spring 8 and the bimetal plate 15 in order to serve as a spacer and to provide thermal insulation between the contact spring 8 and the bimetal plate 15.
Also, a film resistor 17 is disposed on the bottom surface 1a of the support 1. This film resistor 17 is electrically connected to the terminal tabs 3 and 4 via conductive strips 18.
When the bimetal plate 15 is inverted in response to a temperature equal to or greater than the switching temperature and lifts the contact spring 8, electric currents flow only through the film resistor 17, while the support 1 is heated and itself heats the bimetal plate 15; thereby the spring prevents the bimetal plate 15 from returning to the original position that closes the switch.
As described above, because the collar 16 of the pin 12 serves as a spacer and provides thermal insulation between the contact spring 8 and the bimetal plate 15, the bimetal plate 15 is hardly affected at all by the Joule heat generated in the contact spring 8.
Additionally, the invention disclosed in the above International Patent Application Publication No. WO87/03137 is based on a concept that a heat source for operating the bimetal temperature switch (referred to as a temperature switch hereinafter) is externally provided (in other words, the temperature switch is operated as a single unit), and this invention employs a configuration for detecting external hot air.
The above conventional temperature switch contains six problems.
The first problem is that the temperature switch employs a configuration to set a special function so that once the status of the switch changes, the switch does not return to the original state when this switch is connected to an external circuit in series. In other words, this switch does not have a common function for opening and closing in accordance with temperature variations.
The second problem is that this temperature switch does not perform heat detection efficiently because this switch has a low responsiveness to heat, and thus reliability is a problem when this switch is used for controlling the temperature of a hot plate heater that is included in, for example, a hair iron or is used for protecting the hot plate heater.
The third problem is that this switch consists of a large number of parts, and for engaging these parts or mounting these parts on the substrate, operations such as welding, soldering, brazing, caulking, rivet caulking, catching, and the like are often required. Thus, the configurations are complicated, and many steps have to be executed for the assembly.
The fourth problem is that performing caulking requires a highly developed skill because substrates are sometimes broken when performing caulking if the substrates are made of ceramic, leading to a lower yield. However, it is difficult to acquire personnel having such a highly developed skill.
The fifth problem is that the engagement based on catching requires a step of bending an elastic material, and it is impossible to bend an elastic material at a sufficient level so as to cause the catching functions because assembly of the elastic material causes a spring back that is too strong.
The sixth problem is that to form a catching part by bending an elastic material before assembly while also taking the margin of the spring back into consideration requires a step of sliding the catching part of the temperature switch from an end portion to the engagement part in the substrate. This greatly limits the shaping of the substrates and the positioning of engagement parts, thereby decreasing degrees of freedom.
In view of the above problems, it is an object of the present invention to provide a temperature switch that consists of a minimum number of components, that is inexpensive, that is highly responsive to heat detection when being used for a hot plate heater, and that can easily be attached to a substrate made of ceramic or the like.